1. Field of the Invention
The present invention relates to a temperature estimation apparatus for estimating temperature of a motor.
2. Description of the Related Art
Conventionally, a temperature estimation apparatus for estimating motor temperature has been proposed to determine whether or not a motor has overheated. Such a temperature estimation apparatus either estimates motor temperature based on a detection temperature detected by a temperature detection device, or estimates motor temperature based on a motor loss which is sequentially calculated, or performs both estimations.
When motor temperature is estimated based on temperature detected by the temperature detection device, the temperature detection device is disposed in a vicinity of a winding wire (a heat generation source) provided in the motor and then the detection temperature detected by the temperature detection device is estimated as the temperature of the motor.
On the other hand, as described in Unexamined Japanese Patent Publication (Kokai) No. 9-93795, when a temperature of a motor is estimated based on a motor loss, a current passing through the motor is sequentially acquired in each preset sampling period (250 microseconds, for example); a motor loss is sequentially calculated from the current sequentially acquired; and then the temperature of the motor is estimated from the loss sequentially calculated.
Even if a temperature detection device is disposed in a vicinity of a winding wire during production of motors, it is impossible to place the temperature detection device in the same position for all motors having the same shape and made of the same material. Accordingly, placement of the temperature detection device is different for each motor. Such a difference in placement of the temperature detection device causes a detection temperature detected by the temperature detection device to vary even when the amount of heat generated is the same. Such variation in the detection temperature increases in accordance with an increase in a temperature gradient of a periphery of the position of the temperature detection device. For example, when a coolant filling unit of a cooling system is disposed on an exterior side of a motor, the coolant filling unit is disposed in a vicinity of a winding wire in order to enhance a cooling effect. Therefore, when amounts of heat generation of motors are the same, a detection temperature detected by a temperature detection device decreases as the shortest distance between a placement position of the temperature detection device and the coolant filling unit becomes shorter. Therefore, a timing for outputting an alarm for overheat protection is delayed as the shortest distance between the placement position of the temperature detection device and the coolant filling unit becomes shorter. Accordingly, an influence on variation in the detection temperature detected by the temperature detection device due to the difference in the placement position of the temperature detection device for each motor causes a difference in an estimated temperature for each motor. Therefore, it is impossible to accurately estimate the temperature of the motor in order to determine whether or not the motor is overheated.
On the other hand, when a temperature of a motor is estimated based on a motor loss which is sequentially calculated, a current is sequentially acquired in each sampling period to sequentially calculate a motor loss. As a result, a variation of the loss of the motor increases as a variation between a current value acquired in a certain sampling period and a current value acquired in the subsequent sampling period becomes larger. Further, a variation of an estimated temperature increases as a variation of the motor loss becomes larger. Therefore, a variation of current passing through the motor affects a variation in an estimated temperature of the motor, resulting in a disadvantage that it is impossible to accurately estimate the temperature of the motor in order to determine whether or not the motor is overheated.